Rewrite state-cache

[bkchr]

The state cache, found here: https://github.com/paritytech/substrate/blob/master/client/db/src/storage_cache.rs is very error prone.
We have already tried to fix it multiple times, but there still exist bugs. Parachains for example are the ones that suffer from these state cache bugs which mainly lead to storage root mismatches.

The idea is to rewrite the state-cache into an overlay model, so that we just cache all the writes/deletes per block. Then, we can use these overlays to build the "state view" when accessing a particular state. Reads also should still be cached.